Sort example syntax is incorrect

[dfdan]

Package version (if known): current (also according to Discord I hit this in 2021...)

Describe the bug

The example for custom sort orders given in customize/search is incorrect - it only works for me if

you use metadata.title.keyword here:

docs-invenio-rdm/docs/operate/customize/search.md

Line 99 in f5a0702

fields=['metadata.title'],

Steps to Reproduce

1. Follow examples in docs
2. Try and search, get an error

Expected behavior

Docs should be updated, probably this needs a little bit of explanation of when to use .keyword or not.